Here's another good reason to go outside: exposure to dirt may make you smarter.
PhysOrg reports on research suggesting that ingesting Mycobacterium vaccae, a soil bacterium, stimulates neuron growth, increases serotonin levels, decreases anxiety, and increases learning ability -- at least, in mice.
"This research suggests that M. vaccae may play a role in anxiety and learning in mammals," says Matthews. "It is interesting to speculate that creating learning environments in schools that include time in the outdoors where M. vaccae is present may decrease anxiety and improve the ability to learn new tasks."